Project Summary Tooth decay is preventable, yet costly. Spending on dental treatment for children totaled $8.7 billion in 2013? making it the fifth most costly child health condition. Preventive oral health services, like fluoride varnish, can prevent tooth decay in young children. However, fewer than 50% of children have an annual dental visit, likely due to cost, dentists? reluctance to treat young children, and dental workforce shortages. To increase use of preventive oral health services, private health plans and all state Medicaid programs now cover delivery of fluoride varnish in medical offices to all children younger than 6 years. Existing evidence indicates that delivery of fluoride varnish in medical offices increases utilization and reduces decay in publicly insured children. Despite these benefits, few children receive fluoride varnish in medical offices. In 2014, only 5% of eligible children received this service in Massachusetts. Prior studies to identify barriers and strategies have focused on publicly insured children and relied on self-reported provider information, thus a new ?big data? approach is needed to identify provider, practice, community-, and policy-level factors affecting delivery of fluoride varnish in medical offices. Our mixed-methods study will use Massachusetts All Payer Claims Dataset, which includes both medical and dental claims for publicly and privately insured children, to (1) describe children receiving preventive oral health services, (2) estimate the causal impact of mandatory medical coverage on children?s receipt of preventive oral health services, (3) examine the mechanisms affecting delivery of preventive oral health services in medical offices, and (4) use a qualitative positive deviance approach to identify strategies and contextual factors promoting or deterring medical providers? delivery of preventive oral health services.